Influence of a 4'-substituent on the efficiency of flavonol-based fluorescent indicators of β-glycosidase activity

This article presents novel fluorescent probes, based on the excited-state intramolecular proton transfer (ESIPT) phenomenon and flavonols, sensitive to the action of specific glycosidases. 4'-Substituted flavonols were synthesized, using various approaches, and glycosylated with d-glucose, N-acetyl-d-glucosamine and d-glucuronic acid. Evaluation of the β-glycosidase activities was performed in neutral and acidic pH. In all the cases examined, an acidic environment accelerated enzymatic hydrolysis. It was demonstrated that the 4'-chloroflavonyl glycosides of all sugars tested, both in neutral and acidic pH, are the ones most sensitive to the presence of hydrolase. In turn, 4'-dimethylaminoflavonyl glucoside is not sensitive to glucosidase action at all. Generally, the rate of enzymatic hydrolysis increases as the electron-withdrawing nature of the 4'-substituent increases. An exception is the trifluoromethyl group which, in spite of having the most favourable Hammett constant, does not contribute enough to increase the rate of hydrolysis of its glucoside. The presented experimental results are supported by the electrostatic potential (ESP) analysis and related to the mechanisms of glycoside bond enzymatic hydrolysis.

The detection of the precursors of the photorearranged products of 3-hydroxyflavones in selected solvents from UV-visible spectra in situ

Mechanistic studies relating to the photochemistry of 3-hydroxy-2-phenyl-4H-chromen-4-one (3HF) and 6-chloro-3-hydroxy-2-phenyl-4H-chromen-4-one (Cl-3HF) have been reinvestigated in selected solvents. The UV-visible spectra of the photoproduct(s) of 3HF and Cl-3HF have been computed in situ via subtracting the spectra of unreacted substrates, with acetonitrile (ACN) and methanol (MeOH) as solvents. These spectra turn out to be different from the spectra of the corresponding isolated photoproducts: 3-hydroxy-3-phenyl-indan-1,2-dione and 6-chloro-3-hydroxy-3-phenyl-indan-1,2-dione (referred to as dione ). Analyses of the photoproduct(s) via GC-MS show the formation of a single detectable product, i.e., the corresponding dione. On the basis of some experimental observations, it is proposed that the primary photoproduct in situ is 2,3-epoxy-2-hydroxy-1-indanone (referred to as epoxide) instead of dione as reported in previous years. Earlier, epoxide has been proposed to be the intermediate in the mechanism for the formation of dione. This is the first report where the formation of epoxide has been directly detected in the selected solvents. On the other hand, both dione and epoxide (2 : 1) are shown to be formed with MeOH as solvent. The second important finding is that epoxide and dione interconvert in the dark, depending upon the environment. With ACN as solvent, pure dione in the dark is kinetically and partially converted to epoxide. With MeOH as solvent, epoxide is instantly and partially converted to dione until both are in equilibrium. However, a solution of dione in MeOH remains stable in the dark. The photoformation of epoxide is quantitative with ACN as solvent and it is sufficiently stable. It has been further observed that epoxide solutions of 3HF and Cl-3HF in ACN are quantitatively converted into 3-phenylisobenzofuran-1(3H)-one and 6-chloro-3-phenylisobenzofuran-1(3H)-one, i.e., the corresponding phthalides, through the loss of CO when kept in the dark for some days. A mechanism has been proposed where epoxide has been shown to give dione and/or phthalide via selective C-O or C-C bond cleavage in the oxiranyl ring, respectively. The selection of this cleavage depends mainly on the solvent system and the substituents in the parent flavones.

Role of solute-solvent hydrogen bonds on the ground state and the excited state proton transfer in 3-hydroxyflavone. A systematic spectrophotometric study

A detailed account on the photophysics of 3-hydroxyflavone (3HF) in 27 organic solvents is reported. Dual fluorescence of neutral 3HF was observed in protic, polar, and weakly polar solvents, endowed with sufficiently high hydrogen bond accepting and/or donating capabilities. Ground-state solvent-induced 3HF deprotonation was reported in 14 cases. 3HF anion photophysics was investigated, and the deprotonation constant Kdep calculated. Previously reported models (based on solute-solvent intermolecular hydrogen bonds) to explain solvent effects on Excited-State Intramolecular Proton Transfer (ESIPT) and on solvent-induced deprotonation have been re-examined and improved in order to rationalize the observed photophysical behaviour in all the studied solvents. Hydrogen bond donor acidity and hydrogen bond acceptor basicity are shown to be key parameters. The results are discussed in the framework of the use of 3HF as an environment-sensitive fluorescent sensor in several research fields, and as a model system in the study of ESIPT reactions. Solvent effects on 3HF reactivity are also discussed, as the role of the surrounding media on the chemistry of flavonols is an emerging topic in natural product research.

Photoacidity as a tool to rationalize excited state intramolecular proton transfer reactivity in flavonols

Determination of acidic strengths at the electronic ground and excited states of flavonol derivatives.

Theoretical study on the photooxygenation and photorearrangement reactions of 3-hydroxyflavone

The mechanisms of three photodegradation reactions of 3-hydroxyflavone – its photosensitized oxygenation, photooxygenation with ³O₂ and photorearrangement into an indanedione derivative – have been investigated by computing the free energy profiles.

Flavonol-based fluorescent indicator for determination of β-glucosidase activity

A flavonol-based ESIPT fluorescence probe for evaluation of β-glucosidase activity was synthesized and tested for sensitivity to enzymatic cleavage at different conditions.

Single and double intramolecular proton transfers in the electronically excited state of flavone derivatives

3,7-Dihydroxyflavone derivatives containing carbonyl fragments were synthesized. Results of the fluorescent spectroscopy investigations indicate that one of them undergoes Excited State Intramolecular Double Proton Transfer (ESIDPT).